A recently standardized millimeter wave, such as a band of 60 GHz, easily transmits data at rates of gigabits per second (Gbps), using about 2 GHz broadband communication devices without high modulation, but has a strong tendency to travel in a straight line and involves substantial power loss due to characteristics a high frequency wave. Thus, there is a method of using a directional antenna to collect power not in all directions but in a particular direction in order to obtain a high antenna gain.
However, when a line of sight (LOS) is not secured, a signal is transmitted via reflection, and accordingly a distance becomes longer, increasing a free space path loss and adding a loss due to the reflection. Moreover, a penetration loss is more than 20 dB when the LOS is blocked by a person, and a loss due to indoor walls or doors may be greater, so that signals may not be transmitted. Further, in a 60 GHz communication network, a range is limited to less than 10 meters, because an attenuation according to a distance becomes greater in a farther distance despite no a reflection or penetration loss. Thus, there is a high possibility that signals are not transmitted.
A wireless local area network (WLAN) used at home with a distance of more than 10 meters and having a band of less than 5 GHz has a range of tens of meters, and thus a single network covers the distance. However, a 60 GHz communication network does not cover the distance, causing inconvenience to users. Furthermore, since it is difficult to form a single network in rooms or a room and a living room where the LOS is blocked by doors or walls, described above, there is a need for a compensation method.
Thus, communication methods based on directional communication proper to the above band, such as WLAN or wireless personal area network (WPAN), are considered. However, when network conditions are dynamically changed, for example, when the LOS is blocked by a person during transmission of data, a LOS communication is impossible, so a bypass link is used. In this instance, there is a demand for synchronization of a transmitter with a receiver.
Further, the receiver may need to adjust a reception direction of a signal prior to the transmitter in order not to miss signals transmitted by the transmitter through a bypass link, and there is a need for a relay method used in a reserved period regardless of a transmission scheme through the bypass link.